1. Field of the Invention
The present invention relates to an image-reading apparatus and an image-forming apparatus, each having an image sensor, and in particular relates to an image-reading apparatus and an image-forming apparatus, each having a contact image sensor.
2. Description of the Related Art
Recently, in response to demand for reduction in cost and electric power consumption of a light source, in addition to an image scanner using a demagnification optical system constituted of a charge coupled device (referred to below as a CCD) and a lens, which are combined therewith, a contact image sensor (referred to below as a CIS) is increasing to be used, which is constituted of a 1× magnification image-forming optical system of a distributed index lens, such as a SELFOC™ lens; and a CCD line sensor or a CMOS (complementary metal-oxide semiconductor) line sensor; which are combined therewith using a light source, such as a light-emitting diode (referred to below as an LED) or a xenon lamp.
For example, when an original document is read at a speed of about 25 ipm (images per minute) with a color copying machine, whereas an original document surface illuminance of about 30,000 lux is necessary for the demagnification optical system using the CCD, an examined review has been obtained in that the contact image sensor can read the sheet at the same reading speed and substantially the same S/N with an original document surface illuminance of about 3,000 lux, which is 1/10 of the above. The CIS has such an advantage.
The CIS also has a feature that an image scanner can be reduced in thickness, weight, and size by using the CIS. Then, in order to follow this feature, a configuration is proposed, in which signal transmission between an image-signal processing circuit board having an image-signal processing circuit for processing an image signal and the CIS is performed using analog signals.
When the signal transmission is performed with analog signals in such a manner, since the CIS need not have a digital circuit mounted thereon for processing an image signal, the reduction in thickness and weight is enabled. In fact, the inventor tried to mount the digital circuit on the CIS, but abandoned it because of the increase in thickness, width, and weight due to the mounting of the circuit board and components.
By the configuration in that a digital signal is not transferred between the CIS and an image-signal processing section, radio wave noise can be reduced. Recently, it is essential to conform to various regulations against producing radio wave noise of United State Federal Communications Committee (FCC) and Voluntary Control Council for Interference by Information Technology Equipment (VCCI), so that an examined result has been obtained in that analog transmission, which can reduce the radio wave noise, is advantageous more than digital transmission.
In such analog transmission, the CIS and the image-signal processing section are connected therebetween using an inexpensive flexible flat cable (referred to below as an FFC) or flexible print circuit (referred to below as an FPC), so that an image signal from the image sensor can be transmitted in an analog state.
On the other hand, in an image scanner section of a copying machine, in consideration of a life enduring the number of scanning times of about one million and pushing a heavy book document on a glass plate of an original document bed, it has been vital to improve the rigidity of a casing of the image-reading apparatus by making it of a metal. If the casing for accommodating the image-signal processing circuit board having the image-signal processing circuit for operating the apparatus with a high-frequency clock is made of a metal, an effect on the radio-wave noise suppression can also be brought out. Therefore, in the copying machine, wiring of the FFC or FPC and the grounded metallic casing exist together.
However, if the metallic casing and the wiring exist together, the capacitance between the wiring and the metallic casing varies with the movement or deformation of the wiring due to the scanning operation, so that the waveform of the analog signal changes, producing a problem. If such unnecessary changes in the waveform are generated, noise is incorporated in the image signal, resulting in unevenness and stripes in the finally produced images, which deteriorate image quality.
The degree of the deterioration in image quality is no longer negligible, and it is so remarkable as being visually and easily recognizable when reading a halftone chart with an optical density D of 0.3. Therefore, such deterioration in image quality significantly reduces the commodity value.
Such deterioration in image quality is not produced in an image scanner having a resin casing for accommodating the image-signal processing circuit board, and it is specific to an image scanner having a metallic casing.